1. Field of the Invention
The present invention concerns the realization of an amplifier that has differential input and outputs, and more particularly, such an amplifier that is required to function at high frequencies.
2. Discussion of the Related Art
FIG. 1 represents a schematic diagram of a known amplifier that has differential inputs and outputs. This amplifier A is provided with inputs e- and e+ and outputs o- and o+. In order for amplifier A to function with a predetermined gain, there is an associated resistance network that comprises a resistance R1 connected in series with each of the inputs of the amplifier and a resistance R2 connected between each input of the amplifier and its corresponding output. Referring to the input voltages as Vi1 and Vi2 and the output voltages as Vo1 and Vo2, the gain of this amplifier is determined by the value of the resistors. Since, Vo1-Vo2=(-R2/R1).(Vi1-Vi2), the differential gain is equal to -R2/R1.
When the input signal varies around an average value, the output signal will vary around the same average value, which is commonly referred to as the common mode voltage VCM. Furthermore, each of the outputs o+ and o- may, because of the very nature of the differential amplifier, reach their respective minimum and maximum determined values which are close to the high and low voltage supplies of an integrated circuit, in which, is realized the differential amplifier. In order to avoid any distortion in the output signal, while taking advantage of the maximum range of signal excursion, it is desirable that the value VCM is centered as precisely as possible on the aforementioned minimum and maximum values.
In order to obtain a well balanced fully differential amplifier that is capable of operating satisfactorily at high frequencies, various authors, for example K. Nakamura et al., IEEE Journal of Solid-State Circuits, Volume 27, No. 4, April 1992, pages 563-567, have proposed the use of structures such as that illustrated in FIG. 2. These structures, called fully differential folded-cascode amplifiers or fully differential level translator amplifiers, comprise a differential input pair 10, that has differential inputs and outputs, followed by two cross-coupled single ended output differential amplifiers 11 and 12. The characteristics of the output voltages Vo1 and Vo2 as a function of the differential input voltage (Vi1-Vi2) should ideally be such as that illustrated in FIG. 3A, that is to say, that the common mode voltage VCM is perfectly centered at the middle values VCM0 of the extreme values of the voltages Vo1 and Vo2. In fact, it should be noted that in practice the characteristic obtained is rather of the type of that of FIG. 3B, that is, when the difference between the input voltages is zero, the common mode voltage, that is to say the crossover point of the voltage curves of Vo1 and Vo2, instead of being equal to the value VCM0; that is centered at the middle values of the extreme values of the voltages Vo1 and Vo2, it is equal to a value VCM1. It is often found that the value of VCM1 may be up to 20% of the difference between the extreme values of the voltages Vo1 and Vo2.
As illustrated in FIG. 2, in order to overcome this inconvenience, it is proposed in the prior art to detect the value of the common mode voltage VCM by means of a resistive bridge, or equivalent, and to inject the offset value of VCM in the feedback loop so as to correct the functionality of the differential input pair 10.
An example of such a representation is described in an article by M. Banu et al., IEEE Journal of Solid-State Circuits, Volume 23, No. 6, December 1988, pages 1410-1414. However, this method presents the inconvenience of creating an additional loop in the circuit which is all the more critical than the gain-bandwidth product of the amplifier, that is to say the frequency of operation of the amplifier is raised since the common mode loop must have a bandwidth as large as the differential mode loop in order to preserve the global value of the gain-bandwidth product.
An object of the present invention is to provide a fully differential amplifier in which the common mode voltage may be adjusted without a feedback loop.
Another object of the present invention is to provide such an amplifier that is a fully differential amplifier wherein the common mode voltage maybe regulated during its fabrication.